How to use the Alt + right click in edit mode in Ubuntu 17.xx?
You could use Alt + Shift + &lt;Right Mouse Button&gt; instead.
(This shortcut is normally not used by the system.)
This will have the same effect, except the "Shift" key will make it add the new selection to an already existing one.
So what you should do is:
1. Press A (if needed) to remove any old selections.
2. Press Alt + Shift + RMB.